antioxidants Review Biophysical Modulation of the Mitochondrial Metabolism and Redox in Bone Homeostasis and Osteoporosis: How Biophysics Converts into Bioenergetics Feng-Sheng Wang 1,2,†, Re-Wen Wu 3,† , Yu-Shan Chen 1, Jih-Yang Ko 3, Holger Jahr 4,5 and Wei-Shiung Lian 1,2,* 1 Core Laboratory for Phenomics and Diagnostic, Department of Medical Research and Chang Gung University College of Medicine, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung 83301, Taiwan; [email protected] (F.-S.W.); [email protected] (Y.-S.C.) 2 Center for Mitochondrial Research and Medicine, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung 83301, Taiwan 3 Department of Orthopedic Surgery, Chang Gung University College of Medicine, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung 83301, Taiwan; [email protected] (R.-W.W.); [email protected] (J.-Y.K.) 4 Department of Anatomy and Cell Biology, University Hospital RWTH, 52074 Aachen, Germany; [email protected] 5 Department of Orthopedic Surgery, Maastricht University Medical Center, 6229 ER Maastricht, The Netherlands * Correspondence: [email protected]; Tel.: +886-7-731-7123 † F.-S.W. and R.-W.W. contribute to this article equally. Abstract: Bone-forming cells build mineralized microstructure and couple with bone-resorbing Citation: Wang, F.-S.; Wu, R.-W.; cells, harmonizing bone mineral acquisition, and remodeling to maintain bone mass homeostasis. Chen, Y.-S.; Ko, J.-Y.; Jahr, H.; Mitochondrial glycolysis and oxidative phosphorylation pathways together with ROS generation Lian, W.-S. Biophysical Modulation of meet the energy requirement for bone-forming cell growth and differentiation, respectively. Moder- the Mitochondrial Metabolism and ate mechanical stimulations, such as weight loading, physical activity, ultrasound, vibration, and Redox in Bone Homeostasis and electromagnetic field stimulation, etc., are advantageous to bone-forming cell activity, promoting Osteoporosis: How Biophysics bone anabolism to compromise osteoporosis development. A plethora of molecules, including ion Converts into Bioenergetics. channels, integrins, focal adhesion kinases, and myokines, are mechanosensitive and transduce Antioxidants 2021, 10, 1394. https:// mechanical stimuli into intercellular signaling, regulating growth, mineralized extracellular matrix doi.org/10.3390/antiox10091394 biosynthesis, and resorption. Mechanical stimulation changes mitochondrial respiration, biogenesis, dynamics, calcium influx, and redox, whereas mechanical disuse induces mitochondrial dysfunction Academic Editors: Ivan Dimauro, Parisi Attilio and Duranti Guglielmo and oxidative stress, which aggravates bone-forming cell apoptosis, senescence, and dysfunction. The control of the mitochondrial biogenesis activator PGC-1α by NAD+-dependent deacetylase Received: 27 July 2021 sirtuins or myokine FNDC/irisin or repression of oxidative stress by mitochondrial antioxidant Nrf2 Accepted: 25 August 2021 modulates the biophysical stimulation for the promotion of bone integrity. This review sheds light Published: 30 August 2021 onto the roles of mechanosensitive signaling, mitochondrial dynamics, and antioxidants in mediating the anabolic effects of biophysical stimulation to bone tissue and highlights the remedial potential of Publisher’s Note: MDPI stays neutral mitochondrial biogenesis regulators for osteoporosis. with regard to jurisdictional claims in published maps and institutional affil- Keywords: osteoblasts; osteoporosis; biophysical stimulation; mechanosensitive; mitochondria iations. biogenesis; PGC-1α; Fndc5; Irisin; Sirt3; Nrf2 Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. Osteoporosis is characterized by very poor bone quality, including loss in the bone This article is an open access article mineral density, microarchitecture, and mechanical strength, etc., and is the leading etio- distributed under the terms and logical cause of skeletal fragility and dysfunction in postmenopausal women and elderly conditions of the Creative Commons people [1]. Bone-making cells synthesize the bone matrix to form a finely woven mineral- Attribution (CC BY) license (https:// ized microarchitecture. These cells also produce cytokines or couple with bone-resorbing creativecommons.org/licenses/by/ cells, dynamically keeping the bone turnover unfluctuating [2]. 4.0/). Antioxidants 2021, 10, 1394. https://doi.org/10.3390/antiox10091394 https://www.mdpi.com/journal/antioxidants Antioxidants 2021, 10, 1394 2 of 18 Bone-forming cells consume a considerable amount of energy to maintain bone for- mation capacity and balance the remodeling activity. The mitochondrial metabolism is an indispensable process in producing the cellular energy component adenosine triphosphate (ATP), which powers cells for survival and to remain functional. Mitochondrial dysfunction induces reactive oxygen species (ROS) overproduction, aggravating oxidative stress to cellular microenvironment [3]. Promoting the mineral acquisition capacity of bone-making cells by bone anabolic factors, including vitamin D, the Wnt pathway, bone morphogenetic protein, and parathyroid hormone (PTH), etc. facilitates the net gain of bone mass, keeping skeletal tissue strong [4,5]. The Food and Drug Administration (FDA)-approved anti-osteoporosis agents, Ro- mosozumab and Denosumab are found to neutralize sclerostin (a Wnt pathway inhibitor) and osteoclastogenic cytokine RANKL respectively, reducing the risk of osteoporotic frac- ture. Teriparatide, a recombinant PTH (1–34), promotes osteoblastic activity and bone formation to delay menopause or glucocorticoid excess-mediated osteoporosis [6]. We deliver new insights into the biological function of mitochondrial bioenergetics and redox to bone-forming cells for bone tissue integrity. Regular exercise, moderate weight loading, and physical activities, in concert with skeletal muscle, are advantageous to bone health [7]. However, unloading, microgravity, and mechanical disuse are found to dysregulate bone formation and remodeling reactions, accelerating bone mass loss and osteoporosis development [8]. The mechanosensitive na- ture of skeletal tissue prompts the development of various types of biophysical stimulation interventions, which provide axial mechanical or oscillatory force or pulsed stimuli to enhance bone formation in osteoporotic or fractured bones [9]. Bone-making cells adapt to extracellular biophysical stimuli through modulating intracellular signaling transductions to change skeletal metabolism and microenvironment homeostasis [10]. A plethora of molecules, including PIEZOs, TRPVs, integrins, cytoskele- tons, and myokines “sense” biophysical stimuli, activating downstream signaling cascades to regulate growth and differentiation of bone-forming cells, bone regeneration, and os- teoporosis development [11]. The contribution of mechanotransduction to mitochondrial activity and redox in bone-forming cells and to the bone microenvironment warrants a wide scope of review. This review illustrates the role of the mitochondrial metabolism in bone-forming cell function and bone anabolism, the effect of biophysical stimulation on mitochondrial activity and mechanosensitive pathways in bone-forming cells, and the remedial potential of mechanosensitive molecules and mitochondrial metabolism regulators for osteoporosis. 2. The Role of Mitochondrial Homeostasis in Bone-Forming Cell Function Mitochondria are powerhouses in the cellular microenvironment, metabolizing glu- cose, amino acids, and fatty acids and producing ATP and ROS through well-organized tricarboxylic acid (TCA) cycle and electron transfer chains. Mitochondrial bioenerget- ics is indispensable in sustaining the growth, differentiation, and biological function of bone-forming cells in response to anabolic factors, including bone morphogenetic proteins (BMPs), PTH, Wnt, and nitric oxide (NO) [3] (Figure1). This organelle also provides energy to protect cells from extracellular stresses. Mito- chondrial dysfunction induces metabolism repression, electron transfer chain disruption, and redox imbalance, accelerating cellular senescence or apoptotic programs [3]. On the other hand, the disposal of unwanted mitochondrial macromolecules through a sophisti- cated autophagy program, which is referred as mitophagy, maintains intercellular microen- vironment integrity. This dynamic process is required to maintain cellular homeostasis, whereas mitophagy defects induce oxidative stress, thus, accelerating cell dysfunction. Antioxidants 2021, 10, 1394 3 of 18 Figure 1. Schematic diagram showing the glycolysis and oxidative phosphorylation pathways involved in the bone-forming cell anabolism. 2.1. Mitochondrial Bioenergetics and Metabolism Mitochondrial ATP is biosynthesized through glycolysis or oxidative phosphoryla- tion pathways [12]. The mitochondrial energetics pathways involve bone-forming cell growth and differentiation. The inhibition of oxidative phosphorylation by antimycin A, an inhibitor for cellular oxidative phosphorylation, impairs osteogenic gene expression and ex- tracellular matrix synthesis in C3H10T1/2 mesenchymal progenitor cells. Forced oxidative phosphorylation by replacing glucose with galactose promotes osteogenesis. This effect activates β-catenin, a master Wnt signaling component, through an acetyl-CoA-dependent acetylation reaction [13]. Oxidative phosphorylation is a prominent reaction of ATP generation in nondiffer- entiated MC3T3-E1 osteoblasts, whereas glycolysis dominants the bioenergetic activity in